The nucleating agent for polyolefin resins is an additive for increasing the rate of crystallization of polyolefin resins and for improving their optical properties such as clarity and gloss, and their mechanical characteristics such as rigidity and elastic modulus.
For use as such a nucleating agent, several compositions each predominantly composed of rosin and/or a metal salt of rosin-based material have recently been proposed (Japanese Unexamined Patent Publications (Kokai) No. H7-330967, No. H7-331081, No. H8-277343, No. H8-277366).
However, the polyolefin resin nucleating agents described in the above patent literature, namely rosin and/or its metal salts, remain to be improved in terms of the heat resistance, oxidation resistance, moisture barrier properties, and blocking resistance. Furthermore, because of their high volatility and sublimability, those nucleating agents have the disadvantage of poor stability and causing poor workability in the processing of the polyolefin resin. Moreover, when rosins or their metal salts proposed are used alone, the effect of clarifying polyolefin resins is not necessarily fully satisfactory. In the case of a mixture of a rosin and a rosin metal salt, its dispersibility in resin is not good enough so that spots apparently attributable to the undispersed nucleating agent are formed in the resin molded products. Therefore, for the purpose of producing the intrinsic nucleating effects, the following contrivances have been proposed (Japanese Unexamined Patent Publications (Kokai) No. H7-330967, No. H7-331081, No. H8-277343, No. H8-277366).
(1) Melt-kneading with polyolefin resin is performed at least twice.
(2) Two or more rosin acid metal salts of different metal species are used in combination.
(3) At least one compatibilizer selected from the class consisting of rosin glycerol esters, antistatic agents, higher fatty acid metal salts, polyolefin wax, or hydrogenated petroleum resin is added to the nucleating agent.
However, any of those nucleating agents still has a great room for further improvement.